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quential data that must be read in order to trigger the read-‐ahead cache above. The de-‐
fault value, 24, (using the same stripe value as above 128KB), means that the client must
request 3MB of sequential data in order to activate the cache. Setting this value too low
would force the array to re-‐cache over and over as fragmented files occur. Setting it too
high might force it not to cache something that otherwise would benefit the client.
Maximum Read Ahead Commands Outstanding:
While the array will appear to be
sending and receiving data, the client is also sending commands to the array to tell it to
read or write data. The client, for example, might send a request to the array to send back
(read) 1MB of data, however before the array has finished, the client might send a request
to the array to send back another 1MB. This is happening anywhere up to millions of
times per second. This setting controls how many of those commands will be buffered at
a time. The default value of 8 is good for most cases. Setting the number too low may re-‐
sult in jerky playback -‐ i.e. the computer sends a request, the array sends back the data,
then waits for the next request. Setting it too high would just waste memory.
Number of Stripes in Each Read Ahead Request:
This can control the size of each re-‐
quest. The default value is 8 (x 128KB), which is 1MB. This keeps the data coming from
the array at a consistent rate -‐ i.e. if the requests from the client were not limited, the re-‐
quests might be uneven, possibly interrupting playback for other clients.
Enable Random Reads:
The array is capable of applying the read-‐ahead cache to non-‐
sequential sectors/stripes. The default value enables this. If it is disabled, the read-‐ahead
will only apply to sequential reads where the sectors/stripes themselves are sequential.
Cache Flush Percentage Threshold (0-‐100):
This controls how often when writing,
that the cache should write its contents to disk and empty itself. The default value is 10
(%), which means that when the cache is at least 10% full, it should empty. The cache size
which was chosen when the RAID was created has a direct bearing on this setting. For
example, if you used a cache size of 3GB, and this value is set to 10, then the write cache
will flush when it is roughly 300MB full. The default number is fine in most cases. If you
set the number too low, you will disable the effectiveness of the write cache, as it will be
emptying more often. If you make it too high, you risk having to wait for a larger cache
flush.
Maximum Write Back Requests Outstanding:
Just as you can control how many com-‐
mands the read will buffer, you can also control the amount of commands that the write
will buffer. The default value of 8 is good for most cases. Setting the value too low or too
high may result in dropped frames on capture because either you are not allowing the cli-‐
ent computer to send enough write commands, or are accepting too many. Setting the
value too high will waste RAM.
Number of Stripes in Each Write Back Request:
This setting controls a limit on the
amount of cache to use for each write command from a client. The default value is 8,
which is 1MB. This is fine in most cases. Making the value too low would limit the cache
too much. Making it too high would probably just waste RAM.
QOS 8-Bay User Manual
QOS Server is a subsidary of Ocean Tides Productions Ltd. © Copyright 2013
